Theory of the Earth Volume I Part 5

The proof I give is this, That besides the general conformation of those hard bodies, so as to be perfectly adapted to each other's shape, there is, in some places, a mutual indentation of the different pieces of gravel into each other; an indentation which resembles perfectly that junction of the different bones of the _cranium_, called sutures, and which must have necessarily required a mixture of those bodies while in a soft or fluid state.

This appearance of indentation is by no means singular, or limited to one particular specimen. I have several specimens of different marbles, in which fine examples of this species of mixture may be perceived. But in this particular case of the Spanish pudding-stone, where the mutual indentation is made between two pieces of hard stone, worn round by attrition, the softening or fusion of these two bodies is not simply rendered probable, but demonstrated.

Having thus proved, that those strata had been consolidated by simple fusion, as proposed, we now proceed to show, that this mineral operation had been not only general, as being found in all the regions of the globe, but universal, in consolidating our earth in all the various degrees, from loose and incoherent sh.e.l.ls and sand, to the most solid bodies of the siliceous and calcareous substances.

To exemplify this in the various collections and mixtures of sands, gravels, sh.e.l.ls, and corals, were endless and superfluous. I shall only take, for an example, one simple h.o.m.ogeneous body, in order to exhibit it in the various degrees of consolidation, from the state of simple incoherent earth to that of the most solid marble. It must be evident that this is chalk; naturally a soft calcareous earth, but which may be also found consolidated in every different degree.

Through the middle of the Isle of Wight, there runs a ridge of hills of indurated chalk. This ridge runs from the Isle of Wight directly west into Dorsetshire, and goes by Corscastle towards Dorchester, perhaps beyond that place. The sea has broke through this ridge at the west end of the Isle of Wight, where columns of the indurated chalk remain, called the Needles; the same appearance being found upon the opposite sh.o.r.e in Dorsetshire.

In this field of chalk, we find every gradation of that soft earthy substance to the most consolidated body of this indurated ridge, which is not solid marble, but which has lost its chalky property, and has acquired a kind of stony hardness.

We want only further to see this cretaceous substance in its most indurated and consolidated state; and this we have in the north of Ireland, not far from the Giants Causeway. I have examined cargoes of this lime-stone brought to the west of Scotland, and find the most perfect evidence of this body having been once a ma.s.s of chalk, which is now a solid marble.

Thus, if it is by means of fusion that the strata of the earth have been, in many places, consolidated, we must conclude, that all the degrees of consolidation, which are indefinite, have been brought about by the same means.

Now, that all the strata of the mineral regions, which are those only now examined, have been consolidated in some degree, is a fact for which no proof can be offered here, but must be submitted to experience and inquiry; so far, however, as they shall be considered as consolidated in any degree, which they certainly are in general, we have investigated the means which had been employed in that mineral operation.

We have now considered the concretions of particular bodies, and the general consolidation of strata; but it may be alleged, that there is a great part of the solid ma.s.s of this earth not properly comprehended among those bodies which have been thus proved to be consolidated by means of fusion. The body here alluded to is granite; a ma.s.s which is not generally stratified, and which, being a body perfectly solid, and forming some part in the structure of this earth, deserves to be considered.

The nature of granite, as a part of the structure of the earth, is too intricate a subject to be here considered, where we only seek to prove the fusion of a substance from the evident marks which are to be observed in a body. We shall, therefore, only now consider one particular species of granite; and if this shall appear to have been in a fluid state of fusion, we may be allowed to extend this property to all the kind.

The species now to be examined comes from the north country, about four or five miles west from Portfoy, on the road to Huntly. I have not been upon the spot, but am informed that this rock is immediately connected or continuous with the common granite of the country. This indeed appears in the specimens which I have got; for, in some of these, there is to be perceived a gradation from the regular to the irregular sort.

This rock may indeed be considered, in some respects, as a porphyry; for it has an evident ground, which is feld-spar, in its sparry state; and it is, in one view, distinctly maculated with quartz, which is transparent, but somewhat dark-coloured[11].

[Note 11: Plate II. fig. 1. 2. 3.]

Considered as a porphyry, this specimen is no less singular than as a granite. For, instead of a siliceous ground, maculated with the rhombic feld-spar, which is the common state of porphyry, the ground is uniformly crystallised, or a h.o.m.ogeneous regular feld-spar, maculated with the transparent siliceous substance. But as, besides the feld-spar and quartz, which are the const.i.tuent parts of the stone, there is also mica, in some places, it may, with propriety, be termed a granite.

The singularity of this specimen consists, not in the nature or proportions of its const.i.tuent parts, but in the uniformity of the sparry ground, and the regular shape of the quartz mixture. This siliceous substance, viewed in one direction, or longitudinally, may be considered as columnar, prismatical, or continued in lines running nearly parallel. These columnar bodies of quartz are beautifully impressed with a figure on the sides, where they are in contact with the spar. This figure is that of furrows or channels, which are perfectly parallel, and run across the longitudinal direction of the quartz. This is represented in fig. 4. This striated figure is only seen when, by fracture, the quartz is separated from the contiguous spar.

But what I would here more particularly represent is, the transverse section of those longitudinal siliceous bodies These are seen in fig. 1.

2. and 3. They have not only separately the forms of certain typographic characters, but collectively give the regular lineal appearance of types set in writing.

It is evident from the inspection of this fossil, that the sparry and siliceous substances had been mixed together in a fluid state; and that the crystallization of the sparry substance, which is rhombic, had determined the regular structure of the quartz, at least in some directions.

Thus, the siliceous substance is to be considered as included in the spar, and as figured, according to the laws of crystallization proper to the sparry ground; but the spar is also to be found included in the quartz. It is not, indeed, always perfectly included or inclosed on all sides; but this is sometimes the case, or it appears so in the section.

Fig. 5. 6. 7. 8. 9. and 10. are those cases magnified, and represent the different figured quartz inclosing the feld-spar. In one of them, the feld-spar, which is contained within the quartz, contains also a small triangle of quartz, which it incloses. Now, it is not possible to conceive any other way in which those two substances, quartz and feld-spar, could be thus concreted, except by congelation from a fluid state, in which they had been mixed.

There is one thing more to be observed with regard to this curious species of granite. It is the different order or arrangement of the crystallization or internal structure of the feld-spar ground, in two contiguous parts of the same ma.s.s. This is to be perceived in the polished surface of the stone, by means of the reflection of light.

There is a certain direction in which, viewing the stone, when the light falls with a proper obliquity, we see a luminous reflection from the internal parts of the stone. This arises from the reflecting surfaces of the sparry structure or minute cracks, all turned in one direction, consequently, giving that luminous appearance only in one point of view.

Now, all the parts of the stone in which the figured quartz is directed in the same manner, or regularly placed in relation to each other, present that shining appearance to the eye at one time, or in the same point of direction. But there are parts of the ma.s.s, which, though immediately contiguous and properly continuous, have a different disposition of the figured quartz; and these two distinguished ma.s.ses, in the same surface of the polished stone, give to the eye their shining appearance in very different directions. Fig. 3. shows two of those figured and shining ma.s.ses, in the same plane or polished surface.

It must be evident, that, as the crystallization of the sparry structure is the figuring cause of the quartz bodies, there must be observed a certain correspondency between those two things, the alinement (if I may be allowed the expression) of the quartz, and the shining of the sparry ground. It must also appear, that at the time of congelation of the fluid spar, those two contiguous portions had been differently disposed in the crystallization of their substance. This is an observation which I have had frequent opportunities of making, with respect to ma.s.ses of calcareous spar.

Upon the whole, therefore, whether we shall consider granite as a stratum or as an irregular ma.s.s, whether as a collection of several materials, or as the separation of substances which had been mixed, there is sufficient evidence of this body having been consolidated by means of fusion, and in no other manner.

We are thus led to suppose, that the power of heat and operation of fusion must have been employed in consolidating strata of loose materials, which had been collected together and ama.s.sed at the bottom of the ocean. It will, therefore, be proper to consider, what are the appearances in consolidated strata that naturally should follow, on the one hand, from fluidity having been, in this manner, introduced by means of heat, and, on the other, from the interstices being filled by means of solution; that so we may compare appearances with the one and other of those two suppositions, in order to know that with which they may be only found consistent.

The consolidation of strata with every different kind of substance was found to be inconsistent with the supposition, that aqueous solution had been the means employed for this purpose. This appearance, on the contrary, is perfectly consistent with the idea, that the fluidity of these bodies had been the effect of heat; for, whether we suppose the introduction of foreign matter into the porous ma.s.s of a stratum for its consolidation, or whether we shall suppose the materials of the ma.s.s acquiring a degree of softness, by means of which, together with an immense compression, the porous body might be rendered solid; the power of heat, as the cause of fluidity and vapour, is equally proper and perfectly competent. Here, therefore, appearances are as decidedly in favour of the last supposition, as they had been inconsistent with the first.

But if strata have been consolidated by means of aqueous solution, these ma.s.ses should be found precisely in the same state as when they were originally deposited from the water. The perpendicular section of those ma.s.ses might show the compression of the bodies included in them, or of which they are composed; but the horizontal section could not contain any separation of the parts of the stratum from one another.

If, again, strata have been consolidated by means of heat, acting in such a manner as to soften their substance, then, in cooling, they must have formed rents or separations of their substance, by the unequal degrees of contraction which the contiguous strata may have suffered.

Here is a most decisive mark by which the present question must be determined.

There is not in nature any appearance more distinct than this of the perpendicular fissures and separations in strata. These are generally known to workmen by the terms of veins or backs and cutters; and there is no consolidated stratum that wants these appearances. Here is, therefore, a clear decision of the question, Whether it has been by means of heat, or by means of aqueous solution, that collections of loose bodies at the bottom of the sea have been consolidated into the hardest rocks and most perfect marbles[12].

[Note 12: This subject is extremely interesting, both to the theory of the earth, and to the science cf the mining art; I will now ill.u.s.trate that theory, with an authority which I received after giving this dissertation to the Royal Society. It is in the second volume of M. de Saussure's _voyages dans les Alpes_. Here I find proper examples for ill.u.s.trating that subject of mineralogy; and I am happy to have this opportunity of giving the reasoning of a man of science upon the subject, and the opinion of a person who is in every respect so well qualified to judge upon a point of this kind.

The first example is of a marble in the Alps, (_voyages dans les Alpes._) tom. 2. page 271.

"La pate de ces breches est tantot blanche, tantot grise, et les fragmens qui y font renfermes font, les uns blancs, les autres gris, d'autres roux, et presque toujours d'une couleur differente de celle de la pate qui les lit. Ils sont tous de nature calcaire; tels etaient au moins tous ceux que j'ai pus observer; et ce qu'il-y-a de remarquable, c'est qu'ils sont tous poses dans le sens des feuillets de la pierre; on diroit en les voyant, qu'ils ont tous ete comprimes et ecrases dans le meme sens. Cette meme pierre est melee de mica, sur-tout dans les interstices des couches et entre les fragmens et la pate qui les reunit; mais on ne voit point de mica dans les fragmens eux-memes. On trouve aussi dans ces breches des infiltrations de quartz. Cette pierre est coupee par des frequentes fissures perpendiculaires aux plans des couches. On voit clairement que ces fentes out ete formees par l'inegal affaiss.e.m.e.nt des couches, et non par une retraite spontanee: car les morceaux ou fragmens etrangers sont tous partages et coupes net par ces fissures au lieu que dans les divisions naturelles des couches, ces memes fragmens sont entiers et saillans au dehors de la surface. Les noeuds de quartz et les divers crystaux, que renferment les roches feuilletees, presentent le meme phenomene, et l'on peut en tirer la meme consequence; ils font partages dans les fentes, et entiers dans les separations des couches."

He finds those particular strata in the other side of the mountain _col de la Seigne_, and gives us the following observations:

"Plus bas on pa.s.se entre deux bancs de ces memes breches, entre lesquels sont interposees des couches d'ardoises noires et de gres feuilletes micaces, dont la situation est la meme.

"On retrouve encore ces breches vers le has de la descente, au pied de pyramides calcaires dont j'ai parle plus haut. Je trouvai en 1774 de tres-jolis crystaux de roche qui s'etaient formes dans les fentes de cette breche. Il y avoit meme un melange de quartz et de mica qui s'etoit moule dans quelques-une de ces fentes. C'etoit donc une roche semblable aux primitives, et pourtant d'une formation posterieure a celle de la pierre calcaire. Et quel systeme pourroit nous persuader que la nature ne puisse encore produire ce qu'elle a produit autrefois!"

M. de Saussure has here given us an example of a calcareous Braccia, as he calls it, but which is rather a pudding stone, with veins or contractions of the ma.s.s. He does not seem to understand these as consequences of the consolidation of those strata; this, however, is the only light in which these appearances may be explained, when those bodies are thus divided without any other separation in the ma.s.s.

The second example is found in the vertical strata of those mountains through which the Rhone has made its way in running from the great valley of the _Vallais_ towards the lake of Geneva. (Chapitre xlviii.)

"C'est une espece de petrosilex gris, dur, sonore, un peu transparent, qui se debite en feuillets minces parfaitement plans et reguliers. Ces feuillets, ou plutot ces couches, courent a 35 degres du nord par est, en montant du cote de l'ouest sous un angle de 80 degres. Ces couches sont coupees par des fentes qui leur sont a-peu-pres perpendiculaires et qui le sont aussi a l'horizon. Cette pierre s'emploie aux memes usage que l'ardoise, mais elle est beaucoup plus forte et plus durable, parce qu'elle est plus dure et moins accessible aux impressions de l'eau et de l'air.

-- 1047. "Ces petrosilex feuilletes changent peu-a-peu de nature, en admettant dans les interstices de leurs feuillets des parties de feldspath. Ils out alors l'apparence d'une roche feuilletee, quartzeuse et micacee, (_quartzum fornac.u.m W._). Mais cette apparence est trompeuse; car on n'y trouve pas un atome de quartz: toutes les parties blanches qui donnent du feu contre l'acier, font du feldspath; et les parties grise ecailleuses ne font point du mica, ce sont de lames minces du petrosilex dont j'ai deja parle."

Here is evidently what I would call petuntze strata, or porcelane stone, that is, strata formed by the deposits of such materials as might come from the _detritus_ of granite, arranged at the bottom of the sea, and consolidated by heat in the mineral regions. We have precisely such stratified ma.s.ses in the Pentland hills near Edinburgh. I have also a specimen of the same kind, brought from the East Indies, in which there is the print of an organized body. I believe it to be of some coralline or zoophite.

-- 1048. "Cette roche melangee continue jusqu'a ce que le rocher s'eloigne un peu du grand chemin. La, ce rocher se presente coupe a pic dans une grande etendue, et divise par de grandes fentes obliques, a-peu-pres paralleles entr'elles. Ces fentes partagent la montagne en grandes tranches de 50 a 60 pieds d'epaisseur, que de loin semblent etre des couches. Mais lorsqu'on s'en approche, on voit, par le tissu meme de la pierre feuilletee, que ses vraies couches font avec l'horizon des angles de 70 a 75 degre, et que ces grandes divisions sont de vraies fentes par lesquelles un grand nombre de couches consecutives sont coupees presque perpendiculairement a leurs plans. Les ma.s.ses de rocher, comprises entre ces grandes fentes, sont encore divisees par d'autres fentes plus pet.i.tes, dont la plupart sont paralleles aux grandes, d'autres leur sont obliques; mais toutes sont a tres-peu-pres perpendiculaires aux plans des couches dont la montagne est composee."

Here is a distinct view of that which may be found to take place in all consolidated strata, whatever be the composition of the stratum; and it is this appearance which is here maintained to be a physical demonstration, that those strata had been consolidated by means of heat softening their materials. In that case, those stratified bodies, contracting in cooling, form veins and fissures traversing perpendicularly their planes; and these veins are afterwards filled with mineral substances. These are what I have here distinguished as the _particular_ veins of mineral ma.s.ses; things perfectly different from proper mineral or metallic veins, which are more general, as belonging to immense ma.s.ses of those strata; and which had been formed, not from the contraction, but from the disrupture of those ma.s.ses, and by the forcible injection of fluid mineral substances from below. Now these two species of veins, the particular and the general, although occasionally connected, must be in science carefully distinguished; in the one, we see the means which had been employed for the consolidation of the strata; in the other, we see that power by which the strata have been raised from the bottom of the sea and placed in the atmosphere.]

Error never can be consistent, nor can truth fail of having support from the accurate examination of every circ.u.mstance. It is not enough to have found appearances decisive of the question, with regard to the two suppositions which have been now considered, we may farther seek confirmation of that supposition which has been found alone consistent with appearances.

If it be by means of heat and fusion that strata have been consolidated, then, in proportion to the degree of consolidation they have undergone from their original state, they should, _caeteris paribus_, abound more with separations in their ma.s.s. But this conclusion is found consistent with appearances. A stratum of porous sand-stone does not abound so much with veins and cutters as a similar stratum of marble, or even a similar stratum of sand-stone that is more consolidated. In proportion, therefore, as strata have been consolidated, they are in general intersected with veins and cutters; and in proportion as strata are deep in their perpendicular section, the veins are wide, and placed at greater distances. In like manner, when strata are thin, the veins are many, but proportionally narrow.

It is thus, upon chemical principles, to be demonstrated, That all the solid strata of the globe have been condensed by means of heat, and hardened from a state of fusion. But this proposition is equally to be maintained from principles which are mechanical. The strata of the globe, besides being formed of earths, are composed of sand, of gravel, and fragments of hard bodies, all which may be considered as, in their nature, simple; but these strata are also found composed of bodies which are not simple, but are fragments of former strata, which had been consolidated, and afterwards were broken and worn by attrition, so as to be made gravel. Strata composed in this manner have been again consolidated; and now the question is, By what means?

If strata composed of such various bodies had been consolidated, by any manner of concretion, from the fluidity of a dissolution, the hard and solid bodies must be found in their entire state, while the interstices between those const.i.tuent parts of the stratum are filled up. No partial fracture can be conceived as introduced into the middle of a solid ma.s.s of hard matter, without having been communicated from the surrounding parts. But such partial separations are found in the middle of those hard and solid ma.s.ses; therefore, this compound body must have been consolidated by other means than that of concretion from a state of a solution.

The Spanish marble already described, as well as many consolidated strata of siliceous gravel, of which I have specimens, afford the clearest evidence of this fact. These hard bodies are perfectly united together, in forming the most solid ma.s.s; the contiguous parts of some of the rounded fragments are interlaced together, as has already been observed; and there are partial shrinkings of the ma.s.s forming veins, traversing several fragments, but perfectly filled with the sparry substance of the ma.s.s, and sometimes with parts of the stone distinctly floating in the transparent body of spar. Now, there is not, besides heat or fusion, any known power in nature by which these effects might be produced. But such effects are general to all consolidated ma.s.ses, although not always so well ill.u.s.trated in a cabinet specimen.

Thus we have discovered a truth that is confirmed by every appearance, so far as the nature of the subject now examined admits. We now return to the general operation, of forming continents of those materials which had been deposited at the bottom of the sea.